Surface mode enhanced by avoided crossing in microstructure fibers for improved SERS sensing

Photonic crystal fibers (PCFs), providing ingenious microfluidic channels and intense light guidance, have many advantages in the detection of three-dimensional surface-enhanced Raman scattering (SERS). In solid-core PCFs, the light interacts with the sample in the form of an evanescent field, which is weak and limits the SERS detection performance. In this paper, we propose a new mechanism based on avoided crossings between the fiber core mode and the surface mode, which greatly enhances the laser power ratio in the liquid channels and improves the SERS sensitivity. Numerical results show that the intensity in the liquid channels can be increased from 2.8 % to 42 %. To realize this avoided-crossing effect in PCFs experimentally, the silver mirror reaction was optimized which can decorate discrete and densely distributed silver nanoparticles on the hydroxylated surface of the microfluidic channels in less than 15 min. The resulting SERS-active suspended-core PCF has ultra-high sensitivity, with a detection limit of 10−12 M, and an enhancement factor of 1010 for crystal violet solution. It also provides good stability and reproducibility with a relative standard deviation of 4.8 %. This ultra-sensitive and highly reproducible PCF-SERS probe has great potential for application in rapid in-situ detection. •Light interacts with the analyte in the form of surface mode rather than the evanescent field.•The surface mode is enhanced by the avoided crossing effect.•High density Ag nanoparticles were simply formed on the inner surface of the hydroxylated optical fiber.•The SERS performance was greatly improved, with an enhancement factor of 1010 for crystal violet solution.